A prestressed concrete (hereafter referred to as “PC”) has tensioned strand in the concrete at installation, which has improved resistance of crack under tension because of its compression force. Thus using the prestressed concrete can provide improved load bearing structure. Applying a synthetic adhesive resin coating with a surface of the PC strand has been conducted to improve corrosion prevention effect of the tensioned strand in the prestressed concrete.
A method of an electrostatic powder coating or an immersion coating under flowing using a powder coating composition has been generally conducted as a method for coating a PC strand with a synthetic resin. Specifically, a method for electrostatic powder coating of a strand with a partially-cured thermosetting epoxy resin is disclosed in a patent literature 1 (Japan patent publication of JP S59 (1984)-130960 A1). In addition, a method for immersion coating under flowing including heating PC strand with plural single wires mutually twisted, and attaching synthetic resin powder in a untwisted manner, heating to melt the resin powder and returning to the original twisted state is disclosed in a patent literature 2 (Japan patent publication of JP H02 (1990)-033386 A1).
However, an immersion coating under flowing of a strand with a conventional powder coating composition may often provide uneven thickness, which has various film thicknesses in a cross section of PC strand.
On the other hand, an electrostatic immersion coating under flowing, which is a combination coating method of an immersion coating under flowing and electrostatic application to a coating composition in order to improve an attachment of a coating composition, may often provide uneven thickness and stringiness. The stringiness is a phenomenon of spinning a synthetic fiber by melting coating film in a flow direction of air.
In these methods, a film thickness of more than 1200 μm is provided in a thick film part in order to fulfill a minimum film thickness within standards, which leads to high expense because of increase of attachment amount of a powder coating composition. In addition, such thick film may counteract fixing of the strand by a fix tool and sliding when the obtained PC strand is put under stress.
In order to prevent uneven thickness, an electrostatic powder coating method (hereafter referred to as “electrostatic gun powder coating”) can be applied instead of the electrostatic immersion coating under flowing. The coating method however needs installation of multiple spray guns around the strand. The coating method furthermore needs sensitive control of the number of spray guns, amount of discharged coating composition from the spray guns and a discharge direction of the spray guns. Defection of such control may provide increase of used amount of a coating composition or decrease the yield caused by a fault of that thickness, which leads to high expense.    Patent literature 1: Japan patent publication of JP S59 (1984)-130960 A1    Patent literature 2: Japan patent publication of JP H02 (1990)-033386 A1